The present invention relates to an exposure control device for a camera, and more particularly to an exposure control device which employs a magnetic plunger for a driving mechanism of diaphragm members and/or shutter blades.
Conventionally, in a exposure control device for a camera, the magnetic plunger has been used as a part of a driving mechanism for shutter blades and/or diaphragm members. When the magnetic plunger is used as a part of a driving mechanism of the diaphragm member, for example, the magnetic plunger is used for stopping movement of diaphragm members.
An example of such a mechanism includes a lever which prevents or stops movement of diaphragm members. The lever is urged in a certain direction. Further, the lever is provided with a ferromagnetic member which is neutrally attracted by a permanent magnet-of the magnetic plunger. When the exposure control is carried out, the diaphragm members are driven to a predetermined position to define a predetermined aperture. When the diaphragm members have reached the predetermined position, electrical current is supplied to a coil of the magnetic plunger to temporarily demagnetized the magnet force of the magnetic plunger. When the magnetic force of the magnetic plunger is temporarily weakened, the ferromagnetic member of the lever caught by the magnetic plunger is released therefrom, and the lever moves in the urged direction to stop movement of the diaphragm members. After the diaphragm members are moved to the predetermined position to define the predetermined aperture, shutter blades are moved to start exposure. Thus, an appropriate (or desired) exposure condition can be obtained.
In an exposure control device using such a magnet plunger, electrical current is sometimes generated inside the coil of the magnetic plunger due to electrical noise before the electrical current is supplied thereto. Generation of the unintentional electrical current inside the coil may be caused by a self-electromotive effect due to relative oscillation between the coil and the permanent magnet, and/or electrical noises generated by surrounding devices.
The relative movement between the coil and the permanent magnet may occur due to a mechanical impact when mechanical parts of the camera start and/or stop moving. The self-electromotive current generates a magnetic field for the plunger coil. The magnetic field acts to reduce the magnetic force of the permanent magnet in the magnet plunger. As a result, in the above-described example, the lever is incidentally released from the magnetic plunger before the diaphragm members reach the predetermined position. In such a case, an appropriate exposure control cannot be performed.